Compared with the traditional gas turbine fuels of natural gas and crude oil, which consist predominantly of hydrocarbon compounds, the combustible constituents of synthesis gases are substantially CO and H2. Depending on the gasification method and overall plant concept the heating value of the synthesis gas is approximately 5 to 10 times less than the heating value of natural gas. Principal constituents in addition to CO and H2 are inert fractions such as nitrogen and/or water vapor and in certain cases also carbon dioxide. Due to the low heating value it is accordingly necessary to supply gaseous fuel through the burner to the combustion chamber at high volumetric flow rates. The consequence of this is that one or more separate fuel passages must be made available for the combustion of low-calorie fuels such as e.g. synthesis gas. Due to the high reactivity (high flame velocity, large flammability range) of synthesis gases compared to conventional fuels such as natural gas and oil there is a significantly higher risk in respect of flame flashback, which is to say burner damage. For this reason the current practice in industrial gas turbines is to combust synthesis gases exclusively in the diffusion mode of operation. The local high combustion temperatures associated therewith lead to high nitrogen oxide emissions, which are in turn lowered by an additional dilution by means of inert substances such as N2 or water vapor. The additional increase in the fuel mass flow rate associated therewith in turn imposes special requirements on the combustion system and the front-end auxiliary systems.
In the burner according to the prior art—such as described in EP 1 649 219 B1—the synthesis gas is supplied to the combustion chamber by way of an annulus passage arranged around the burner axis. In this case the gas upstream of the burner nozzle is conducted through a nozzle ring present in the burner nozzle and having boreholes inclined at an angle, a circumferential velocity component being applied to the gas. This means that in the prior art a relatively low Mach number is superimposed on the synthesis gas directly at the nozzle. Associated therewith there also exists, due to the low fuel momentum, only a relatively low intensity in terms of the mixing with the combustion air surrounding the annular fuel flow both internally and externally. An additional factor militating against rapid mixing of the fuel with the combustion air is the geometric embodiment of the annular gap with a relatively large gap width and correspondingly large mixing path.
The nozzle ring of EP 1 649 219 B1 having boreholes inclined at an angle was chosen in particular for synthesis gases having a relatively high heating value in order to achieve a sufficiently high pressure loss at the nozzle for acoustic stability, without substantially changing the main dimensions. However, this embodiment has aerodynamic disadvantages. Accordingly, discrete jets are generated which cannot be homogenized to a sufficient extent on the path available up to the burner outlet, thus leading to increased NOX emissions. Furthermore, a considerable total pressure loss occurs due to the flow separations inside and upstream of the nozzle, such that said lost momentum is subsequently not available as mixing energy.